This invention is directed toward rubber vulcanizates having improved tear strength. More particularly, the present invention is directed toward tires and tire components that are produced from vulcanizable compositions of matter that contain polyolefins. The polyefins are functionalized and have been found to increase the tear strength of vulcanizates without deleteriously impacting the mechanical properties of the vulcanizates.
Off road or heavy truck tires are often subjected to rough road conditions that produce repetitive, localized high pressure pounding on the tire. These stresses can cause fatigue fracture and can lead to crack formation and growth. This degradation of the tire has also been referred to as chipping or chunking of the tread surface or base material.
In an attempt to prevent this degradation, it is known to add reinforcements such as carbon black, silicas, silica/silanes or short fibers to tire compositions. Silica has been found advantageous because of its ability to deflect and suppress cut prolongation, and silanes have been added to bind the silica to unsaturated elastomers. The fibers that have been added include nylon and aramid fibers.
It is also known that the addition of polyolefins to rubber compositions can provide several beneficial properties. For example, low molecular weight, high density polyethylene, and high molecular weight, low density polyethylene, are known to improve the tear strength of polybutadiene or natural rubber vulcanizates. In the tire art, It has also been found that polyethylene increases the green, tear strength of carcass compounds and permits easy extrusion in calendering without scorch. Polypropylene likewise increases the green strength of butyl rubber. Polypropylene, has also been effective in raising the static and dynamic modulus of rubber, as well as the tear strength of the rubber.
Although the addition of polyolefins to rubber compositions is known to provide several beneficial effects, the addition of polyolefin to tire recipes has, heretofore, had a deleterious affect on the mechanical, wear, and hysteresis properties of tires, as well as handling and ride comfortability of tires.
Accordingly, there remains a need in the art to improve the tear strength of rubber vulcanizates, especially those deriving from tire compositions, without sacrificing the other properties of vulcanizates, tire components or tires.
It is therefore an object of the present invention to provide a tire component having increased tear strength, where the tire component is less susceptible to chipping and chunking, without substantially impacting the mechanical and wear properties of the tire component.
It is another object of the present invention to provide a tire component having increased tear strength, where the tire component is less susceptible to chipping or chunking, without substantially impacting the hysteresis properties of the tire component.
It is yet another object of the present invention to provide a vulcanizate having increased tear strength, where the vulcanizate is less susceptible to chipping and chunking, without substantially impacting the mechanical and wear properties of the vulcanizate.
It is still another object of the present invention to provide a vulcanizate having increased tear strength, where the vulcanizate is less susceptible to chipping or chunking, without substantially impacting the hysteresis properties of the vulcanizate.
It is also an object of the present invention to provide vulcanizable compositions of matter that will give rise to a cured product having increased tear strength, where the cured product is less susceptible to chipping and chunking, without substantially impacting the mechanical and wear properties of the cured product.
It is another object of the present invention to provide vulcanizable compositions of matter that will give rise to a cured product having increased tear strength, where the cured product is less susceptible to chipping or chunking, without substantially effecting the hysteresis properties of the cured product.
It is yet another object of the present invention to provide a tire having increased tear strength without substantially impacting the mechanical and wear properties of the tire at high temperatures.
It is still yet another object of the present invention to provide a vulcanizate having increased tear strength without substantially impacting the mechanical and wear properties of the vulcanizate after heat aging.
At least one or more of the foregoing objects, together with the advantages thereof over the known art relating to tire components and compositions for making the same, which shall become apparent from the specification that follows, are accomplished by the invention as hereinafter described and claimed.
In general the present invention provides a tire having improved tear strength including at least one component comprising: a vulcanized elastomer; and up to about 35 parts by weight functionalized polyolefin per one hundred parts by weight rubber.
The present invention also provides a vulcanizable composition of matter comprising: an elastomer; up to about 35 parts by weight functionalized polyolefin per one hundred parts rubber; and up to about one hundred parts by weight of a reinforcing filler per one hundred parts by weight rubber.
The present invention further provides a vulcanizate prepared by a process comprising the steps of: preparing a vulcanizable composition of matter that includes an elastomer and a functionalized polyolefin; and vulcanizing the composition of matter with at least one vulcanizing agent.
It has now been found that the addition of functionalized polyolefin to vulcanizable compositions of matter that are useful for making tires provides for tires and tire components having increased tear strength without substantially affecting the mechanical, wear, and hysteresis properties of the tire rubber. Notably, the mechanical properties of the tire components are not substantially degraded after heat aging by the addition of the functionalized polyolefin. Accordingly, the present invention contemplates vulcanizable compositions of matter, tire recipes, vulcanizates, tire components and tires containing functionalized polyolefin. The practice of the present invention is especially useful in base stock recipes, but inasmuch as the increase in tear strength does not deleteriously impact the wear, mechanical, and hysteresis properties of the rubber, the practice of the present invention may also be applied to the tread and sidewall stocks of tires. Furthermore, it should be understood that the practice of the present invention is believed to be especially advantageous for off-road or heavy-duty truck tires, although it is believed that the practice of the present invention will improve other tires such as passenger tires.
The functionalized polyolefins that are useful in this invention include functionalized polypropylene and functionalized propylene-ethylene copolymers. The propylene-ethylene copolymers may simply be referred to as copolymers. In general, the functionalized polyolefins include those polyolefins that contain at least one moiety as a functional group. These moieties can include, for example, those that derive from maleic anhydride, acrylic acid, and epoxides. Maleic anhydride functionalized polyolefins are most preferred.
Generally, the polyolefins should contain from about 0.05 to about 3 percent by weight of the functionalized moiety. More preferably, the polyolefins should contain from about 0.1 to about 2 percent by weight of the functionalized moeity, and even more preferably from about 0.15 to about 0.5 percent by weight of the functionalized moiety.
The functionalized polyolefins that are useful in practicing this invention are, for the most part, commercially available. These commercially available functionalized polyolefins can be prepared by a number of techniques. For example, maleic anhydride can be grafted to a polypropylene homopolymer or copolymer in the presence of organic peroxide either in the melt, solid state, or in solution. The most common method employed is the melt or solid-state processes. These processes may also be referred to as reactive extrusion. For further information on the functionalization of polypropylene or propylene-ethylene copolymers with maleic anhydride by using reactive extrusion techniques, one can refer to Reactive Extrusion Principals and Practice, Reactive Extrusion: A Survey of Chemical Reactions of Monomers and Polymers During Extrusion Processing, pp.75-198, by Xanthos (1992 Hanser Publishers), and Molecular Characterization of Maleic Anhydridexe2x80x94Functionalized Polypropylene, Journal of Polymer Science, pp. 829-842, by Roover, et. al. (1995 John Wiley and Sons, Inc.), both of which are incorporated herein by reference.
The molecular weight of the polyolefin polymers and copolymers used in this invention can vary. Indeed, the molecular weights of commercially available polymers and copolymers vary. It is, however, preferred that the molecular weight of the polyethylene polymers and copolymers employed be from about 100,000 to about 500,000, preferably from about 150,000 to about 400,000, and even more preferably from about 175,000 to about 400,000, as determined by using standard GPC analysis with polystyrene as the standard. Generally, the molecular weight distribution (Mw/Mn) should be less than about 4.5, preferably less than about 4.0, and even more preferably less than about 3.8.
With respect to the polymeric backbone of the functionalized polypropylene, most polypropylene homopolymers that are commercially produced have an isotatic microstructure. The propylene-ethylene copolymers can be a random or block copolymers. Preferably, the copolymers will contain some polyethylene crystals. The copolymers should include a major amount of polypropylene or propylene units and only a minor amount of polyethylene or ethylene units. Specifically, the copolymers should contain less than about 40 percent by weight polyethylene or ethylene units. Preferably, the copolymers should contain from about 1 to about 30 percent by weight polyethylene or ethylene units, more preferably from about 1.5 to about 25 percent by weight polyethylene or ethylene units, and even more preferably from about 2 to about 23 percent by weight polyethylene or ethylene units.
As noted above, most of the functionalized polyolefins that are useful in practicing this invention are commercially available. For example, maleic anhydride functionalized polypropylene is available from the Exxon Chemical Company of Houston, Tex., under the tradename EXXELOR. Specific EXXELOR products,include EXXELOR PO 1015 and 1020. These modified polypropylenes can be purchased at a variety of molecular weights. It should be understood that many commercially available functionalized polypropylenes contain some amount of ethylene or ethylene units. Usually, this amount is less than about 5 weight percent. Functionalized polypropylene and propylene-ethylene copolymers are also available from Elf Atochem of Philadelphia, Pa., under the tradename PPC, CA1000, or OE707. OE707 is a propylene-ethylene copolymer that contain from about 20 to about 25 percent by weight polyethylene. Still further, functionalized polypropylene is available from Uniroyal Chemical Co., Inc. of Middlebury, Conn. under the tradename Polybond 3001, 3002, or 3150.
According to the present invention, functionalized polyolefin is added to a vulcanizable composition of matter that is useful for fabricating tires. Generally, the functionalized polyolefin is added in an amount up to about 35 parts by weight per one hundred parts by weight rubber (phr). Preferably, the functionalized polyolefin is added in an amount from about 5 to about 30 parts by weight phr, more preferably from about 10 to about 25 parts by weight phr, and even more preferably from about 15 to about 22 parts by weight phr.
Although functionalized polyolefins are added to vulcanizable compositions of matter that are useful for fabricating tires, practice of this invention does not alter the type or amount of other ingredients typically included within these vulcanizable compositions of matter. Accordingly, practice of this invention is not limited to any one particular vulcanizable composition of matter or tire compounding stock.
Typically, these vulcanizable compositions of matter include rubber component that is blended with reinforcing fillers and at least one vulcanizing agent. These compositions typically also include other compounding additives. These additives include, without limitation, accelerators, oils, waxes, scorch inhibiting agents, and processing aids. As known in the art, vulcanizable compositions of matter containing synthetic rubbers typically include antidegradants, processing oils, zinc oxide, optional tackifying resins, optional reinforcing resins, optional fatty acids, optional peptizers, and optional scorch inhibiting agents.
These vulcanizable compositions are compounded or blended by using mixing equipment and procedures conventually employed in the art. Preferably, an initial masterbatch is prepared that includes the rubber component and the reinforcing fillers, as well as other optional additives such as processing oil and antioxidants. According to this invention, it is preferred to add the functionalized polyolefin during preparation of the initial masterbatch. Once this initial masterbatch is prepared, the vulcanizing agents are blended into the composition. This vulcanizable composition of matter can then be processed according to ordinary tire manufacturing techniques. Likewise, the tires are ultimately fabricated using standard rubber curing techniques. For further explanation of rubber compounding and the additives conventionally employed, one can refer to The Compounding and Vulcanization of Rubber, by Stevens in Rubber Technology Second Edition (1973 Van Nostrand Reihold Company), which is incorporated herein by reference.
The elastomers that are typically employed within vulcanizable compositions of matter that are useful for making tires include both natural and synthetic elastomers rubbers. For example, these elastomers include, without limitation, natural rubber, synthetic polyisoprene rubber, styrene/butadiene rubber (SBR), polybutadiene, butyl rubber, neoprene, ethylene/propylene rubber, ethylene/propylene/diene rubber (EPDM), acrylonitrile/butadiene rubber (NBR), silicone rubber, the fluoroelastomer, ethylene acrylic rubber, ethylene vinyl acetate copolymers (EVA) epichlorohydrin rubbers, chlorinated polyethylene rubber, chlorosulfonated polyethylene rubbers, hydrogenated nitrile rubber, tetrafluoroethylene/propylene rubber and the like. As used herein, the term elastomer or rubber will refer to a blend of synthetic and natural rubber, a blend of various synthetic rubbers, or simply one type of elastomer or rubber. Also, the elastomers that are useful in practicing this invention include any of the various functionalized elastomers that are conventionally employed in the art of making tires. Inasmuch as the preferred embodiments of the present invention are directed toward off-road and heavy truck tires, it is preferred to employ natural rubber and SBR with natural rubber being most preferred.
The reinforcing agents, such as carbon black or silica, typically are employed in amounts ranging from about 1 to about 100 parts by weight per 100 parts by weight rubber (phr), with about 20 to about 80 parts by weight (phr) being preferred, and with about 40 to about 80 parts by weight (phr) being most preferred. The carbon blacks may include any of the commonly available, commercially-produced carbon blacks, but those having a surface area (EMSA) of at least 20 m2/g and more preferably at least 35 m2/g up to 200 m2/g or higher are preferred. Surface area values used in this application are those determined by ASTM test D-1765 using the cetyltrimethyl-ammonium bromide (CTAB) technique. Among the useful carbon blacks are furnace black, channel blacks and lamp blacks. More specifically, examples of the carbon blacks include super abrasion furnace (SAF) blacks, high abrasion furnace (HAF) blacks, fast extrusion furnace (FEF) blacks, fine furnace (FF) blacks, intermediate super abrasion furnace (ISAF) blacks, semi-reinforcing furnace (SRF) blacks, medium processing channel blacks, hard processing channel blacks and conducting channel blacks. Other carbon blacks that may be utilized include acetylene blacks. Mixtures of two or more of the above blacks can be used in preparing the carbon black products of the invention. Typical values for surface areas of usable carbon blacks are summarized in the following table.
The carbon blacks utilized in the preparation of the rubber compounds used may be in pelletized form or in unpelletized flocculent mass. Preferably, for more uniform mixing, unpelletized carbon black is preferred.
With respect to the silica fillers, the vulcanizable compositions of the present invention may preferably be reinforced with amorphous silica (silicon dioxide). Silicas are generally referred to as wet-process, hydrated silicas because they are produced by a chemical reaction in water, from which they are precipitated as ultrafine, spherical particles. These particles strongly associate into aggregates that in turn combine less strongly into agglomerates. The surface area, as measured by the BET method, gives the best measure of the reinforcing character of different silicas. Useful silicas preferably have a surface area of about 32 to about 400 m2/g, with the range of about 100 to about 250 m2/g being preferred, and the range of about 150 to about 220 m2/g being most preferred. The pH of the silica filler is generally about 5.5 to about 7 or slightly over, preferably about 5.5 to about 6.8.
When employed, silica can be used in the amount of about 1 part to about 100 parts by weight per 100 parts of polymer (phr), preferably in an amount from about 5 to about 80 phr. The useful upper range is limited by the high viscosity imparted by fillers of this type. Usually, both carbon black and silica are employed in combination as the reinforcing filler. When both are used, they can be used in a carbon black:silica ratio of from about 10:1 to about l:2. Some of the commercially available silicas that may be used include: Hi-Sil(copyright) 215, Hi-Sil(copyright) 233, and Hi-Sil(copyright) 190, produced by PPG Industries. Also, a number of useful commercial grades of different silicas are available from a number of sources including Rhone Poulenc. Typically, a coupling agent is added when silica is used as a reinforcing filler. One coupling agent that is conventionally used is bis-[3(triethoxysilyl) propyl]-tetrasulfide, which is commercially available from Degussa, Inc. of New York, N.Y. under the tradename S169.
In addition to the advantageous feature of the present invention noted above, the cost of producing tires, especially off-road tires, can be significantly reduced by employing the formulations according to the present invention. Because functionalized polyolefins can be added to tire formulations or recipes without deleteriously impacting the ultimate properties of the tires, the use of functionalized polyolefins yields significant cost savings.
In order to demonstrate the practice of the present invention, the following examples have been prepared and tested as described in the Experimental Section disclosed hereinbelow. The examples should not, however, be viewed as limiting the scope of the invention. The claims will serve to define the invention.